1. Field of the Invention
The present invention relates to a recording control system for a recording apparatus such as a facsimile machine, and more particularly to a recording control system for a recorder which attains a uniform record line density when a sheet is fed by a stepping motor.
2. Related Background Art
When a stepping motor is driven intermittently at a rotation speed of 800 pps, for example, a pulse rate is increased at the start of drive of the motor from 400 pps to 600 pps and then to 800 pps to attain acceleration control, or conversely the motor is decelerated to stop.
Such a motor control system is, disclosed in U.S. Pat. No. 4340848. Such a control is appropriate for a drive control for driving the motor at a high speed for a relatively long period at a constant time interval such as the feed of a record sheet or transport of a carriage in a conventional printer. However, in paper feed drive in a facsimile machine in which the motor is driven intermittently and at non-constant period and the number of steps per drive is as small as four or eight steps, full acceleration and deceleration are not attained and extra time is required for the acceleration or the deceleration.
The assignee of the present invention has proposed in the U.S. patent application Ser. No. 372,822 to measure an interval of drive triggers for the stepping motor and control a phase excitation energy or a phase excitation time in accordance with the interval so that the rotation of the stepping motor is smoothened.
However, even in a facsimile machine which uses such a stepping motor drive system, it is not fully attained to keep a record line density of the recorder uniform or constant.
In the prior art recorder, a plurality of strobe signals are successively sent to a thermal recording head of the facsimile machine simultaneously with the sending of the trigger to the stepping motor. A plurality of excitation pulses and strobe signals are generated for each trigger pulse in order to rotate the motor as smoothly as possible. In the prior art recorder, the trigger signal is in synchronism with the excitation pulses or the strobe signals and a linear control is usually made. Assuming that the phase excitation is to be advanced by four steps for each motor trigger, the four phase excitation currents of the motor are equal and the intervals are also equal as shown in FIG. 5. The intervals of strobing are also equal accordingly.
A rotor of the motor has an inertia which increases further when the motor is loaded. Accordingly, in actuality, the rotation angle of the motor tends to be delayed with respect to the excitation signal at the beginning. When the motor is to be stopped, the inertia acts reversely so that it tends to overshoot. FIG. 5 shows the movement. A broken line shows an ideal case of the motor rotation. As shown in FIG. 5, during the generation of the strobe signals, that is, during the dot recording, the amount of paper feed is not uniform along the direction of paper feed. As a result, the record line density is not constant and a record quality is poor. When a time period T.sub.2 from the fourth excitation pulse to the next trigger pulse is sufficiently long, the motor has been stopped when the next trigger pulse is applied. As a result, the paper movement is again ununiform as shown in FIG. 5.